In the following description the term “turbine” is used to refer to rotary engines having a rotating part and a stator part force coupled by a fluid medium such as water or gas. Of particular interest for the present invention are axial turbines comprising radially arranged fixed stator blades or vanes alternating with radial arrangements of moving rotor blades. Movements are generally defined herein as movements relative to a casing or housing.
The field of designing and manufacturing large turbines has been dominated by a bespoke approach to design and manufacturing. This approach makes each turbine an individual item customized for a specific power plant or operator. In order to reduce efforts and costs involved in adapting a turbine design to a new set of specifications, attempts have been made in the past to standardize at least parts of the turbine to achieve a more modular and therefore more readily changeable design.
German published patent application no. DE 44 25 352 A1 describes for example a steam turbine having a cast housing, which is constructed as a standard casing to cover a number of different variants. The standard casing according to the DE '352 patent application has several extraction ports, which can be selectively opened according to a given turbine variant. After the casting of the standard housing all extraction ports are closed. For the selective opening of the extraction ports part of the wall are drilled out to form the ports as required.
The casings of large turbines are typically manufactured as two separate parts, i.e., a bottom and a top half. These halves are bolted together on site after the inner parts of the turbine such as the rotor, moving and stationary blades or diaphragms, seals etc. have been put in place. Though split, each half of the casing can still weigh 100 tons or more.
Given the complexity and the costs involved in casting a casing for a turbine, particularly for a large steam turbine, it is seen as an object of the present invention to provide turbine casings, designs of turbine casings and methods of manufacturing casings which reduce the amount of changes required when moving from one set of specifications to another. A particular problem the present invention addresses is the change required to adapt a given type of turbine to different frequencies of the electric power grid and hence to different rotational speeds.